1. Field of the Invention
This invention relates to a process for making aryl isocyanates such as aryl mono-, di- and polyisocyanates by thermally decomposing aryl urethanes at a temperature between 175.degree. C. and 600.degree. C. in the presence of a catalyst, present in a heterogeneous phase, the catalyst being a metal selected from the group consisting of zinc, aluminum, titanium, iron, chromium, cobalt and nickel. More particularly, the invention relates to the use of a catalyst with a large surface area.
2. Description of the Prior Art
It is part of the current state of the art that N-substituted urethanes can be thermally decomposed in a gas or liquid phase into isocyanates. The thermal decomposition simultaneously produces various undesirable secondary reactions, such as the decarboxylation reaction of urethanes, which may be accompanied by the formation of primary and secondary amines and olefins, the reaction of an isocyanate and a urethane to form an allophanate, the reaction of an isocyanate and an amine to form a urea, and the polymerization of isocyanates into a uretdione and an isocyanurate.
According to data in German Published Application 19 44 719 (British Pat. No. 1,247,451), the thermal decomposition of urethanes in the vapor phase is carried out at temperatures of 400.degree. C. to 600.degree. C. in the presence of a Lewis acid catalyst with the isocyanate and the alcohol being separated by fractional condensation. The vapor phase in this case is defined such that the reaction mixture, possibly including a solvent, is present in the vapor phase following the decomposition independent of feeding as gaseous, liquid or solid urethane. Toluene diisocyanate, for instance, is produced by the pyrolysis of toluene-2,4-diethylurethane in the presence of ferric chloride. Drawbacks of this reaction include low yield combined with considerable quantities of a polymeric byproduct, the decomposition of the catalyst and corrosion of the reaction equipment. German Published Application No. 24 10 505 (U.S. Pat. No. 3,870,739) describes a process where the urethane is decomposed at a temperature of 350.degree. C. to 550.degree. C. and a pressure of less than the (m+1) multiple of the isocyanate vapor pressure of the isocyanate product in a catalyst-free pyrolysis zone within 15 seconds. Drawbacks of this process are that a large quantity of heat must be quickly added to the powdered urethane for the endothermal decomposition, and the separation of a solid polymer byproduct makes the implementation of a continuous process more difficult.
The thermal decomposition of urethanes in the liquid phase is described, for instance, in German Application No. 24 21 503 (U.S. Pat. No. 3,962,302) and German Application No. 25 30 001 (U.S. Pat. No. 3,919,280). According to German Application No. 24 21 503, the urethanes are dissolved in an inert solvent such as alkylbenzene, linear and cyclic hydrocarbons, and/or phthalic acid esters, and are decomposed at atmospheric pressure or above at a temperature of from 175.degree. C. to 350.degree. C. The resultant isocyanate and alcohol are separated and isolated by means of the solvent as entraining agent and/or by using an inert gas as entraining agent. According to German Application No. 25 30 001, higher molecular, substituted or unsubstitied aliphatic, cycloaliphatic, or aromatic hydrocarbons, ether, esters or ketones are used as reaction medium. Only distillation is mentioned for isolating the decomposition products with isocyanate, alcohol and entraining agent being distilled overhead whereas the reaction medium remains as bottom fraction.
For the manufacture of aromatic isocyanates according to German Published Application No. 26 35 490, the urethanes are brought in contact with a solution of at least one metal ion such as ions of copper, zinc, aluminum, tin, titanium, vanadium, iron, cobalt and nickel as catalysts which is dissolved in a solvent having a boiling point of 200.degree. C. in a metal concentration of at least 0.001 percent by weight relative to the solvent at temperatures of 150.degree. C. to 350.degree. C. under reduced pressure. The resultant decomposition products are isolated by fractional condensation.
In accordance with the above-mentioned processes, urethanes, depending upon their structure, can be transformed into isocyanates with, in part, very good yields. These publications do not describe, in the form of examples, the manufacture of mixtures of diphenylmethane diisocyanates and polyphenyl polymethylene polyisocyanates (crude MDI) from the corresponding urethane mixtures (crude MDU). Other than the described isocyanates, crude MDI is not completely distillable by using solvents as entraining agents and can therefore not be isolated as described above from the catalyst, solvent and nonreacted raw materials and impurities.
An object of this invention is to improve the thermal decomposition of aryl urethanes into aryl isocyanates, particularly of high molecular aryl-di- and/or -polyurethanes into aryl-di- and/or -polyisocyanates eliminating the above-mentioned drawbacks.